Article 220-Cooking Equipment Calculations, Part VI

Specifications for calculating branch-circuit, feeder and service loads are in Article 220 of the National Electrical Code. This article is divided into four parts: I. General, II. Feeders and Services, III. Optional Calculations for Computing Feeder and Service Loads, and IV. Method for Computing Farm Loads. Part 1 contains general provisions and a number of branch-circuit calculations. After executing the calculation procedures in accordance with Part I, the results can either be used to size branch circuits, or can be added to other applicable demand factors permitted by Parts II, III or IV to size feeders or services. Part II contains specifications for calculating feeders and services. Optional calculations for computing feeder and service loads are in Part III. Part IV contains methods for computing farm loads.

While most of the procedures in Part II are used along with calculations from Part I, some feeder (or service) calculations will not require any calculations from Part I. For example, a panelboard is needed to supply power to 15 household ranges that will be used in culinary arts classes. The only load on this panel will be household electric ranges, and therefore the only calculation procedure needed is from 220.19 and Table 220.19. Last month’s Code In Focus continued a discussion of calculation methods for household electric ranges and other cooking appliances. This month, the discussion concludes the series on cooking equipment calculations.

While most of the provisions in 220.19 pertain to finding loads on feeders or services, the fourth note under Table 220.19 specifies how to calculate branch-circuit loads. As discussed last month, it is permissible to calculate the branch-circuit load for one range in accordance with Table 220.19. The second sentence in the fourth note states that the branch-circuit load for one wall-mounted oven or one counter-mounted cooking unit (cooktop) shall be the nameplate rating of the appliance. While the branch-circuit load for one range can be calculated at less than nameplate rating, the branch-circuit load for one oven or one cooktop cannot. For example, what size 75 C, copper conductors are required to feed an 8.6kW, 240V cooktop? Since the branch-circuit load must not be less than the nameplate rating, the branch-circuit load for one 8.6kW cooktop is 8.6kW or 8,600W. Although the load is as much as some ranges, because it is a cooktop, the branch-circuit load must be the nameplate rating. To determine the minimum conductor size, the branch-circuit ampacity must be known (Watts/volts = Amperes). The conductor ampacity must be rated for at least 36A (8,600/240 = 35.8 = 36). The minimum size 75 C, copper conductors required for an 8.6kW, 240V cooktop is 8 AWG (See Figure 1).

The branch-circuit load for one wall oven must be the nameplate rating of the appliance. Although not specifically mentioned, the branch-circuit load for a double wall oven is also the nameplate rating of the appliance. For example, what size 60 C, copper conductors are required to feed a 7.4kW, 240V double wall oven? The branch-circuit load for this wall-mounted oven is 7.4kW or 7,400W. The conductor ampacity must be rated for at least 31A (7,400/240 = 30.8 = 31). The minimum size 60 C, copper conductors required for a 7.4kW, 240V double wall oven is 8 AWG (See Figure 2). Note, this example is asking for 60 C copper conductors, not 75 C.

The third sentence in the fourth note under table 220.19 contains the last branch-circuit load calculation. The branch-circuit load for a counter-mounted cooking unit and not more than two wall-mounted ovens can be calculated by adding the nameplate rating of the individual appliances and treating the total as equivalent to one range. In this type of installation, the cooktop and oven(s) must be located in the same room and supplied from a single branch circuit. For example, what is the branch-circuit load for one 7.4kW, 240V cooktop and one 3.6kW, 240V wall oven? The household cooking appliances are supplied from a single branch circuit and located in the same room. The first step is to add the two appliances. Calculate the demand load as if calculating for one range at 11kW (7.4 + 3.6). By applying Table 220.19, an 11kW range has a maximum demand (in Column C) of 8kW or 8,000W (See Figure 3).

Where two wall-mounted ovens and one cooktop are supplied from a single branch circuit and located in the same room, the three appliances can be added together and treated as one range. For example, what is the branch-circuit load for one 7.4kW, 240V cooktop and two 3.6kW, 240V wall ovens? The household cooking appliances are supplied from a single branch circuit and located in the same room. The branch-circuit conductors will terminate in a junction box inside the cabinet under the cooktop. The first step is to add the three appliances. Calculate the branch-circuit demand load as if calculating for one range at 14.6kW (7.4 + 3.6 + 3.6). Because it is over 12kW, apply the first note under Table 220.19. The maximum demand in Column C must be increased 5 percent for each additional kilowatt of rating or major fraction thereof by which the rating exceeds 12kW. Since .6 is a major fraction, round the 14.6 up to 15kW. A 15kW range exceeds 12kW by 3kW (15 - 12 = 3). Since Column C must be increased 5 percent for each additional kilowatt of rating above 12, the maximum demand in Column C must be increased by 15 percent (3 x 5% = 15%). The increased amount is 1.2kW (8kW x 15% = 1.2kW). This increased amount must be added to the Column C demand load (8 + 1.2 = 9.2kW). The branch-circuit load for one 7.4kW, 240V cooktop and two 3.6kW, 240V wall ovens is 9.2kW or 9,200W.(See Figure 4). Derating the nameplate the load (in accordance with Table 220.19) is permitted only with a cooktop and one wall-mounted oven, or a cooktop and two ovens. Derating is not permitted when one branch circuit supplies two ovens and no cooktop.

Up to now, the demand load calculations in 220.19 have applied to household electric cooking equipment in dwelling units. Unless meeting the requirement in the last note under Table 220.19, load calculations for kitchen equipment in other than dwelling units must comply with 220.20 and Table 220.20. As stated in the fifth note, the demand loads in Table 220.19 also apply to household cooking appliances rated over 13/4kW and used in instructional programs. Household electric cooking equipment installed in other than dwelling units and not used in instructional programs must be calculated in accordance with 220.20.

Provided the equipment is household cooking equipment and used in instructional programs, the type of occupancy is not limited to dwelling units. For example, an existing high school classroom is being converted into a culinary arts classroom. Fifteen 12kW, 240V household electric ranges will be installed. A separate panelboard will be installed to supply the ranges. What is the load on the feeder for these ranges? Although the ranges are not in a dwelling, because the household cooking appliances will be used in instructional programs, the load can be calculated from Table 220.19. The maximum demand in Column C for 15 12kW ranges is 30kW (See Figure 5). EC

MILLER, owner of Lighthouse Educational Services, teaches classes and seminars on the electrical industry. He is the author of “Illustrated Guide to the National Electrical Code” and NFPA’s “Electrical Reference.” He can be reached at 615.333-3336, charles@charlesRmiller.com or www.charlesRmiller.com.